This invention relates to the new chemical synthesis and new use of metal tetraiodomercurates as infrared detectors in the range of 2 to 10 microns.
Galium arsenide and mercury cadmium telluride are currently used for infrared detectors in detection and visualization equipment in the range of 2 to 10 microns, but these materials are both expensive and highly toxic.
The metal tetraiodomercurates disclosed herein are a series of chemically synthesized materials that can serve as high resolution, lower cost, infrared detectors to replace galium arsenide and mercury cadmium telluride in many applications, including infrared spectrophotometers and radiometers. The infrared properties of these chemical compounds were discovered in the process of developing new thermochromic pigments for energy conserving coatings. These compounds are easily synthesized, processed and purified, and can be used over a wide variety of temperature ranges. The compounds can be pressed into disks as well as grown into crystals, thus reducing the cost of production over other materials. Silver tetraiodomercurate was previously reported as a thermochromic material, that is, the color changes from yellow to red by heating over a range of about 50 degrees C. to 100 degrees C. [K. Funke, Prog. Solid State Chem., 11, 345-402 (1976)]. The color reverts to yellow when this silver compound cools to room temperature. Because of its thermochromic properties it was investigated as a pigment for paints that change color as a function of temperature. It has also been reported [J. A. A. Ketelar, Trans. Faraday Soc., 34, 874-882 (1938)] that this silver compound is a non-conductor or insulator below 50 degrees C., but conducts electricity almost as well as metals, when the compound is heated above 50 degrees C. However, there have not been any reports correlating the optical properties to the electronic properties. During the investigation it was deduced that these materials might have infrared detector properties after examining the changes of optical absorption spectra (or band gap energies) over a temperature range of 0 degrees C. to 100 degrees C. Further evidence came from luminescence studies in the visible and infrared ranges during the investigation. Because of the optical and electronic correlations observed for the silver tetraiodomercurate, the originally reported synthesis in U.S. Pat. No. 2,892,798 issued June 30, 1959, was improved and new metal tetraiodomercurates were synthesized, which are useful as infrared detectors.